The mitochondrial DNA is 16 times more prone to oxidative damage and evolves 10-20 times faster than nuclear DNA. Point mutations and deletions in this mitochondrial genome give rise to a wide range of diseases which cause mitochondrial dysfunction. Point mutations in the mitochondrial genome either maternally inherited or age related give rise to many mitochondrial dysfunctional diseases such as Leber's hereditary optic neuropathy and neurogenic muscle weakness, ataxia, and retinitis pigmentosa. Deletions in the mitochondrial genome give rise to Kearn- Sayre syndrome, Pearson marrow pancreas syndrome and ocular myopathy. These point mutations and deletions occur during replication by the DNA polymerase gamma. Enzymatic analysis of the DNA polymerase gamma has revealed that this polymerase is extremely sensitive to nucleotide analogs as compared to the nuclear DNA replicative polymerases. These analogs include the commonly used antiviral drugs such as AZT and dideoxynucleotides. Patients treated with AZT develop a ragged-red fiber myopathy associated with a reduction in mitochondrial DNA levels. How the mitochondrial DNA polymerase makes point and deletions mutations and what structural properties set this polymerase apart from the nuclear DNA polymerases to give rise to its inhibition patterns is poorly understood. It is our goal to gain an appreciation of the structural relationship of this polymerases and how it relates to other well characterized polymerases. Based on the homology between the S. cerevisiae DNA polymerase gamma and the bacterial DNA polymerases, we have cloned the DNA polymerase gamma genes and cDNA from S. pombe, D. melanogaster and Homo Sapiens. Alignment of these amino acid sequences with the S. cerevisiae DNA polymerase gamma shows that the S. cerevisiae DNA polymerase gamma differs by having an additional 200 amino acids in the C-terminus. The genes for these mitochondrial DNA polymerases have been mapped to their corresponding chromosomes. Monospecific polyclonal antibodies have been raised against overexpressed polypeptides of the human DNA polymerase gamma. These antibodies recognize and immunoprecipitate a 140 kDa protein from mitochondrial extracts with polymerase gamma like activity. Most interestingly, the human DNA polymerase gamma gene contains a CAG trinucleotide repeat in the first exon of the coding sequence. This repeat is expanded and altered in some DNA repair deficient cells.